Frequency Analysis of Low Flow in Dez River Basin

Drought is one of the natural hazards which have serious effects on human life, and environment. This phenomena has a complicated mechanism that its nature had been little-known. Accurate estimation of low flow as an index in water resources management has a great importance. Because of the complicated nature of low flows, a few numbers of studies conducted in this field of study, and the frequency analysis of low flow is of interest to researchers. Therefore, in the present study, the frequency of low flows in the Dez river basin with different durations (7, 10, 15, 30, 60 and 95 days) were done using the distribution functions of the Normal (NOR), Log-Normal (LN), Pearson Type III (P3), Exponential (EXP), Gamma (GAM), Generalized Extreme Value (GEV), Nakagami (NAK), Rayleigh (RAY), Logistic (LOG), Generalized Logistic (GLOG), Generalized Pareto (GPA) and Weibull (WEI). Before fitting the statistical distribution functions on low flow series, the homogeneity and stationarity of low flow series in the Dez river basin were investigated by Augmented Dickey - Fuller (ADF) and modified Mann-Kendall tests, respectively. The results of these testsconfirmed the homogeneity and stationarity of used data series. In the next step, above mentioned distribution functions were fitted on low flow series and the goodness of fit were evaluated by normalized root mean square error and Nash- Sutcliffe criteria. The results reveal that the LOG and GEV distribution functions were the best fitness on low flow series in the Dez river basin. The Normal distribution has also good performance in estimating the low flow and because of its easier application can be considered as an alternative for estimating low flow in the Dez river basin by acceptable error.